This invention relates to a circuit arrangement provided with a voltage-current converter, comprising
a differential amplifier provided with
a first input terminal for connection to a reference voltage source for generating a reference voltage Vref,
a second input terminal for connection of a reference resistor Rref, and
an output,
means I for generating a first current through the reference resistor Rref, and
a current amplifier for generating a second current and provided with an input coupled to the output of the differential amplifier.
Such a circuit arrangement forms part of the integrated circuit UBA2020. In the known circuit arrangement, the current amplifier is a source follower built up from a series arrangement of a transistor and the reference resistor, the output of the differential amplifier being connected to a control electrode of the transistor. The reference resistor thus forms part of the current amplifier, the means I are formed by the transistor and the supply voltage of the source follower. The transistor also forms part of the current amplifier, and the first and second currents are identical. During operation of the circuit arrangement, the output of the differential amplifier keeps the potential at the control electrode of the transistor at a level such that the signal present at the first input of the differential amplifier is substantially equal to the signal present at the second input of the differential amplifier. It is achieved thereby that the relation Vref=Rref*Iref is substantially complied with. Vref here is the reference voltage, Rref the resistance value of the reference resistor, and Iref the current flowing through the reference resistor. The current Iref in the known circuit arrangement is at the same time the current generated by the voltage-current converter. The reference resistor in the known circuit arrangement is not realized within the integrated circuit but is a discrete component which is connected to the second input terminal via a pin of the integrated circuit. The known integrated circuit is mostly used in an electronic lamp ballast comprising a bridge circuit and is designed for controlling this bridge circuit. The bridge circuit generates a high-frequency square-wave voltage whose amplitude is usually of the order of 100 V during lamp operation, and the point where this voltage is available is usually comparatively close to the integrated circuit. The printed conductor track, and the pin of the integrated circuit with which the connection between the reference resistor and the second input terminal is realized together form a parasitic capacitance. The high-frequency square-wave voltage generated by the bridge circuit causes a high-frequency interference signal via this parasitic capacitance, which signal is superimposed on the voltage across the reference resistor. Such a high-frequency interference signal also influences the voltage between the control electrode and the main electrode of the transistor to the reference resistor. As a result, a high-frequency interference signal of comparatively great amplitude is also present in the current generated by the current amplifier. The operation of the circuit arrangement is adversely affected thereby.
It is an object of the invention to provide a circuit arrangement comprising a voltage-current converter wherein the current generated by this voltage-current converter suffers from only a comparatively small amount of interference.
According to the invention, a circuit arrangement as described in the opening paragraph is for this purpose characterized in that the differential amplifier is provided with a low-pass filter, in that the current amplifier on the one hand and the means I and the reference resistor on the other hand exclusively comprise mutually separate components, and in that the circuit arrangement is in addition provided with means II coupled to the current amplifier and to the means I for influencing the first current in dependence on the second current.
According to the invention, the current amplifier on the one hand and the means I and the reference resistor on the other hand exclusively comprise mutually separate components, i.e. the current amplifier on the one hand, and the means I and the reference resistor on the other hand do not have any components in common.
When the circuit arrangement is in operation, the signal present at the output of the differential amplifier is maintained at a level such that the signals present at the first and second input terminals of the amplifier are substantially equal. It is achieved thereby that the relation Vref=Iref*Rref is substantially complied with, so that the amplitude of the first current is substantially equal to Vref/Rref. The signal present at the output of the differential amplifier is also present at the input of the current amplifier and determines the amplitude of the second current generated by this current amplifier. The amplitude of the first current (Iref) is influenced by the means II in dependence on the amplitude of the second current. The second current thus has an amplitude which is determined by the amplitude of the first current and by the means II during stationary operation of the circuit arrangement. The second current is the current generated by the voltage-current converter. If a high-frequency interference signal is present and superimposed on the voltage across the reference resistor during operation of the circuit arrangement, this interference signal will not be present in the current amplifier as well, because the reference resistor does not form a part of the current amplifier. Since the differential amplifier is provided with a low-pass filter, said high-frequency interference signal present at the second input terminal only gives rise to a further high-frequency interference signal of comparatively small amplitude which is superimposed on the signal at the output of the differential amplifier. As a result, little interference is also present at the input of the current amplifier and in the current generated by the current amplifier. Owing to this reduced amount of interference in the current generated by the voltage-current converter, a detrimental effect on the operation of the circuit arrangement owing to interference occurs to a comparatively low degree only.
The low-pass filter may comprise, for example, an ohmic resistor and a capacitance.
In an advantageous embodiment of a circuit arrangement according to the invention, the means II comprise a current mirror for generating a current whose amplitude is substantially equal to the amplitude of the second current, and the means I are formed by a coupling between an output of the current mirror and the reference resistor. In this advantageous embodiment, the first current is derived from the second current by the current mirror, and the first and second currents substantially have the same amplitude. Since current mirrors are widely used in integrated circuits for generating from a given current a number of further currents with substantially the same amplitude, this advantageous embodiment is especially suitable for implementation in an integrated circuit.
Favorable results were obtained with circuit arrangements according to the invention wherein the current amplifier is a source follower. The source follower is a comparatively simple and inexpensive type of current amplifier.
It was found that a further suppression of the interference can be achieved when the reference resistor is shunted by capacitive means. The capacitive means here serve as a filter for the high-frequency interference signal.
It was also found that the circuit arrangement is highly suitable for being constructed at least in part as an integrated circuit.